PARKLAND, Fla. - Terrified students and staff at Marjory Stoneman Douglas High School huddled under desks and hid in closets Wednesday afternoon as gunshots rang out across campus, a former student reportedly pulling the trigger.

Officials from the Broward County Sheriff's Office said 19-year-old Nikolas Cruz was responsible for the attack that left 17 people dead and more than a dozen others wounded. Armed with an AR-15 and multiple magazines, Cruz entered the school and opened fire on student and staff, according to authorities.

Cruz was arrested without incident shortly before 4 p.m. at a location off campus, deputies said.

Broward County Sheriff Scott Israel said Wednesday evening that authorities have already begun to delve into Cruz's social media posts and online activity. He described that content as "disturbing."

He said Cruz was a former Marjory Stoneman Douglas High School student who was expelled for disciplinary reasons, although the exact details of that expulsion were not immediately available.

One teenage boy, who was not identified, told reporters outside the school that Cruz was known for having multiple guns and being a "troubled kid."

"Honestly, a lot of people were saying it was going to be him, stuff like that, we actually, a lot of kids threw jokes around like that, that he was going to be the one to shoot up the school. It turns out that everyone predicted it. That's crazy," the teenager said.

Math teacher Jim Gard, who taught Cruz last year, told the Miami Herald school administrators warned that Cruz had made threats in the past.

“We were told last year that he wasn’t allowed on campus with a backpack on him,” Gard told the newspaper. “There were problems with him last year threatening students, and I guess he was asked to leave campus.”

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News 6 partner WPLG reports that Cruz "liked to kill small animals" and was known to have racist ideologies. In social media posts, he posed with guns and knives oftentimes with his face covered by a bandana or Army hat.

Broward County Public Schools Superintendent Robert Runci said he was not aware of any threat being made prior to the Valentine's Day shooting.

“There’s gonna be a thorough investigation. Typically you see in these situations that there potentially could’ve been signs out there. I would be speculating at this point if there were. But we don’t have -- we didn’t have any warnings, there weren’t any phone calls or threats that were made,” Runci said.

Anyone with information about Cruz or the shooting is asked to call the FBI at 1-800-Call-FBI. For information on how to help the shooting victims and survivors, click here.